1. Field of the Invention
The present invention relates to non-aqueous inkjet inks improved for dispersion stability and suitable for use in manufacturing decorative laminates by single pass inkjet printing.
2. Description of the Related Art
Pigment dispersions are usually made using a dispersant. A dispersant is a substance for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. Dispersants are generally surface-active materials having an anionic, cationic or non-ionic structure. The presence of a dispersant substantially reduces the dispersing energy required. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersing operation due to mutual attraction forces. The use of dispersants also counteracts this re-agglomeration tendency of the pigment particles.
The dispersant has to meet particularly high requirements when used for inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of the pigment particles through the nozzles of the print head, which are usually 10 to 50 micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided during the standby periods of the printer.
Polymeric dispersants generally contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed, and in a spatially separate part of the molecule, the polymeric dispersants have polymer chains compatible with the dispersion medium, thus stabilizing the pigment particles in the dispersion medium. Typical polymeric dispersants include graft copolymer and block copolymer dispersants.
In aqueous inkjet inks, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium.
The preparation of good thermally stable dispersions with submicron particles is much more difficult for non-aqueous inkjet inks, such as solvent based, oil based and radiation curable inkjet inks. The pigments are especially difficult to disperse when they have a non-polar surface.
These dispersing problems have lead to the design of very specific polymeric dispersants wherein the anchor groups are pigment derivatives. For example, EP 0763378 A (TOYO INK) discloses a pigment composition including a non-aqueous type pigment dispersing agent having a portion which has a high affinity with a pigment and which has at least one type selected from the group consisting of an organic dye, anthraquinone and acridone only at a terminal end or at both terminal ends of at least one polymer selected from a linear urethane polymer and a linear acrylic polymer, and a pigment.
Another approach for dispersing pigments with non-polar surfaces in non-aqueous dispersion media is changing the surface to a more polar surface by addition of compounds known as dispersion synergists. A dispersion synergist is a compound that promotes the adsorption of the polymeric dispersant on the surface of the pigment. It is suggested that the synergist should possess the pigment structure substituted by one or more sulfonic acid groups or ammonium salts thereof.
U.S. Pat. No. 4,461,647 (ICI) discloses a dispersion of a pigment in an organic liquid containing a water-insoluble asymmetric disazo compound including a central divalent group free from acidic and other ionic substituents linked through azo groups to two monovalent end groups characterized in that one end group, the first, is free from acidic and other ionic substituents and the other end group, the second, carries a single substituted ammonium-acid salt group.
Although these dispersion synergists work fine for some pigments, many other pigments cannot be dispersed to an acceptable quality in a non-aqueous medium. This is, for example, the case for diketopyrrolo-pyrrole pigments, for which it is difficult to obtain stable non-aqueous pigment dispersions, especially stable non-aqueous inkjet inks.
U.S. 2004/0122130 (CHANG ET AL.) discloses a photo-curable pigment type inkjet ink composition containing Pigment Red 254, but the ink requires the presence of water and a reactive surfactant.
The dispersion quality and stability can also be influenced by the composition of the dispersion medium, however this also influences the behavior of inkjet droplets jetted upon the ink-receiver surface.
Many types of solvents and specific combinations of solvents have been disclosed. EP 1528086 A (SEIKO EPSON) discloses an oil-based inkjet ink including at least 50% of a mixed solvent of a lactone-type solvent and a polyoxyethyleneglycol dialkyl ether. It was discovered by the present inventor that further improvements in dispersion stability and jetting characteristics, such as latency, were particularly required for single pass inkjet printing. For certain inkjet applications, it would also be desirable to make inks lacking a noxious solvent such as lactone.
For consistent image quality, inkjet inks require high dispersion stability capable of dealing with high temperatures (above 65° C.) during transport of the ink to a customer, jetting at elevated temperatures and changes in the dispersion medium of the inkjet ink during use. These changes in the dispersion medium can occur, for example, by evaporation of solvent and increasing concentrations of humectants, penetrants and other additives at the nozzles during the summer or when the printer is not jetting ink for a long time (i.e., latency) and the inkjet print-heads are filled with ink, e.g., over the weekend.
Inkjet printing provides large flexibility in the production of decorative laminates, but high printing speed is required to obtain acceptable productivity. This is possible with single pass inkjet printing if the inkjet printing process is so reliable that during a print job no maintenance is required. Inkjet inks having very high dispersion quality and stability and exhibiting minimal latency problems are therefore required. Aqueous inkjet inks generally exhibit latency problems due to the relative high volatility of water. Diketopyrrolo-pyrrole pigments have been a preferred choice in non-inkjet printing inks, such as, e.g., gravure inks, to print images on a decorative paper used in the manufacturing process of decorative laminates for imitating a wooden floor. These pigments have been dispersed to acceptable dispersion quality and stability in aqueous inkjet inks but not in non-aqueous inkjet inks.